A specific group of reactive polymers possessing oxidation-reduction properties covers electron-exchangers or redox-polymers. These are polymers having macromolecules containing groups capable of entering reversible oxidation-reduction transformations.
Redox-polymers can be prepared by different methods such as polymerization, polycondensation, grafting of polymer chains, polymeranalogous transformations, as well as by way of saturation of ion-exchange resins (ionites) with chemically active compounds possessing reduction-oxidation properties.
Redox-polymers are divided into soluble and insoluble ones. The insoluble (cross-linked) polymers are advantageous in that they may be used as reagents in a chemical reaction and then filtered-off. These polymers can be charged into a column and employed for differential counter-current reactions; the final product is of sufficient purity, thus necessitating no additional purification.
Insoluble redox-polymers are produced mainly as irregularly shaped particles. The most optimal shape is spherical, since it offers minimal resistance to the liquid flow in columns.
The rate of reduction-oxidation reactions in redox-polymers is limited by diffusion of the reagents inside the granules. In insoluble redox-polymers the oxidation-reduction processes are slowed-down with increasing degree of the polymer cross-linking. At a relatively large size and comparatively small specific area, granules of redox-polymers swell only slightly due to the presence of reticulation, the rate of diffusion is decreased and, the rate of oxidation-reduction reactions is retarded.
Redox-polymers of macroporous structure can be obtained by introducing during the synthesis of redox-polymers, special additives (cross-agents) compatible with the starting monomers. Macroporous redox-polymers feature higher kinetic characteristics, though their oxidation-reduction capacity is substantially lower.
Another disadvantage of synthetic redox-polymers is their insufficient osmotic stability and low mechanical strength.
Losses of redox-polymers due to cracking of grains along existing cracks resulting from crushing the synthetic resin or from shrinkage thereof during thermal treatment, in certain cases, are as high as 15-17%.
To eliminate cracking of polymer particles, a more flexible structure of an electron-exchanger is required. All these disadvantages are absent with oxidation-reduction fibers. Indeed, there is a substantially more developed surface area of fibers enriched to a maximum possible extent with active functional groups in the superficial layer, a quick wetting ability and a high capillarity ensure a higher rate of processes occurring with fibrous materials as compared to granulated materials. Furthermore, the effective size of grains of granulated redox-polymers is about 0.40-0.60 mm, whereas the cross-sectional dimension of the majority of reactive fibrous materials is 20-30 times smaller and equal to 0.02-0.03 mm. Consequently, the path of diffusion of the reagents in fibrous materials is also 20-30 times shorter. This fact is the main reason for the substantially higher kinetics of reactive fibers as compared to granulated materials. It is also essential that the majority of modified fibers with reactive groupings possess a high porosity frequently attaining 100-200 m.sup.2 /g.
However, reduction-oxidation (redox) fibers, likewise non-woven fabrics based thereon, feature certain disadvantages, namely: when charged into a column, they become rather rapidly clogged thus sharply increasing the hydrodynamic resistance of the filtering bed.
To overcome this disadvantage, it is necessary that the filtering bed possess elasticity of flexible foamed plastics.
Currently known is a process for producing polyurethane foamed plastics containing ionic groups (cf. U.S. Pat. No. 3,988,268; Cl. 260-2.5, 1976). Amphoteric foamed plastics are produced from the reagents containing cationic and anionic groups. Thus, known in the art is the manufacture of polyurethane foamed plastic by reacting isocyanates such as 1-methyl-2,4-diisocyanate with organic compounds such as ricin polyglycol ether. The mixture is heated to a temperature of 170.degree. C., maintained for three hours and then cooled to room temperature. The disadvantages of these materials reside, in particular, in that they have an insignificant content of ionic groups and are not suitable for the use, for this very reason, in processes of chemisorption and oxidation-reduction. They are intended solely for plant growing.
Also known in the art are ion-exchange foamed materials (cf. U.S. Pat. No. 3,867,319; 1975; U.S. Pat. No. 3,947,387; 1976; Cl. 260-2.5R) which are prepared by foaming a polymer produced in the presence of a volatile polar compound serving as a plasticizer for ionic groups. The polymer contains 0.4 to 10 mol.% of graft acid groups, especially sulpho groups. The material comprises sulphonated polystyrene. The product manufactured by this process has a low exchange capacity (the amount of ionic groups, in particular sulpho groups, is 0.2 to 20 mol. %) and a low mechanical strength. Furthermore, such materials are rigid and brittle.
U.S. Pat. No. 3,094,494; Cl. 260-2.1, 1963, teaches ion-exchange cellular materials consisting of a foamed polyurethane serving as a polymeric matrix and a filler, namely a synthetic ion-exchange resin, employed in an amount of from 0.5 to 160 parts by weight per 100 parts by weight of the polymeric matrix. To produce such materials, 100 parts by weight of polypropylene glycol oligomer (prepared by heating 2 parts by weight of a mixture of 100 g of polypropylene glycol with the molecular mass of 2,000 and 35 parts by weight of toluenediisocyanate/isomeric mixture of 80:20/) are added with 67 parts by weight of a finely divided ion-exchange resin based on sulphonic acid (sulphonated styrene and divinylbenzene in its sodium form) and intermixed form a uniform composition, whereafter the resulting homogeneous mass has added to it a mixture of 2.4 parts by weight of water, 1 part by weight of methylmorpholine, 0.6 part of dimethylpolysiloxane (silicone oil) and is intermixed until foaming occurs. The materials produced by this process are flexible, elastic, and gas- and liquid-permeable. However, under the effect of working solutions the finely-divided ion-exchange resin is washed out of the material, thus substantially affecting its exchange capacity during operation and reducing the life time of such materials. Furthermore, the material is characterized by relatively low kinetic characteristics.
Due to these features it is inefficient to use such ion-exchange cellular materials and resin-filled cellular foams for alleviation of such problems as protection of environment from pollution. Due to low values of exchange capacity, hydrophobic character and insufficient rate of exchange and oxidation-reduction reactions, their use cannot ensure purification of waste waters from harmful substances to permissible concentrations. Furthermore, the possibility of varying properties of the materials for the purpose of widening the scope of their applications is hindered or totally excluded.